CHAPTER 14-2, INTERMEDIATE MESODERM: UROGENITAL SYSTEM ZOO3603C Epithelialization and de-epithelialization in somites of a chick embryo  A formed somite is surrounded by epithelial cells.  Epithelialization occurs first at the posterior edge of the somite.  F-actin accumulation in somites after the separation. N-cadherin vs E-cadherin  Cadherin: Calcium-dependent cell adhesion molecules.  N-cadherin was first described in the CNS of the mouse and chicken  N-cadherin is grouped with E-, P- and R-cadherins into a subfamily.  N-cadherin is strongly expressed in the neural ectoderm, where the expression of E-cadherin in the epidermal ectoderm.  N-Cadherin tends to be expressed in mesenchymal cell types, whereas E-cadherin is expressed in epithelial cell types.  In the progression of many cancers, for very often E-cadherin expression is lost in metastastic cells, while the expression of N-cadherin is elevated. Epithelialization in somite  Epithelialization ≠ become epithelial cell.  Somite = mesodermal cells = mesenchymal cells  Packing somite cells needs the adhesion molecules.  Adhesion molecule in mesenchymal cell is N-cadherin.  Epithelialization of the somites need N-cadherin, not E-cadherin Derivatives of the somites Characteristics of somite cells  The commitment of the cells within a somite occurs relatively late.  Any of early somite cells can become any of the somite-derived structures.The major lineages of the amniote mesodermFour regions of the trunk mesoderm at neurulation 1. Chordamesoderm: Notochord inducing neurulation and establishing the anterior-posterior body axis. 2. Paraxial mesoderm (or somitic mesoderm): Somites producing bone, muscle, cartilage and dermis in the back 3. Intermediate mesoderm: Urogenital system consisting kidneys, gonads and their associated ducts. 4. Lateral plate mesoderm: Circulatory system consisting heart, blood vessel and blood cells.The major lineages of somite Somite Sclerotome Myotome Syndotome Vertebrae and rib cartilage Musculature of the back, ribs and limbs Tendons and vascular cells Dermatome Dermis of the back Ventral-medial side Dorsal side Dermamyotome Endothelial cells Diagram of a transverse section through the trunk of a chick embryo on days 2–4 2-day chick embryo 3-day chick embryo Ventral-medial cells undergo mitosis, and become sclerotome. Sclerotome cells lose their adhesion and migrate toward the neural tube. Remaining epithelial portion, dermamyotome. Sclerotome become vertebrae and rib cartilage Diagram of a transverse section through the trunk of a chick embryo on days 2–4 4-day chick embryo Late 4-day chick embryo Dorsal layer of dermamyotome, dermatome. Dermis of the back Muscle-forming region, primary myotome. Musculature of the back, ribs and limbs Diagram of a transverse section through the trunk of a chick embryo on days 2–4 Late 4-day chick embryo The inter-costal musculature in the rib through back. Myoblasts form primaxial muscles at near neural tube. The lateral plate-derived dermis. Myoblasts form Abaxial muscles at far from neural tube. Between these two, lateral-somitic frontier. Determination of the sclerotome, dermatome & myotome Determination of the sclerotome  The destiny of a somitic region depends on LOCATION.  Sclerotome specification by paracrine factor, Sonic hedgehog secreted from the notochord and the neural tube floor plate.  Sclerotome differentiation by Pax1, a transcription factor.  Sclerotome cells express I-mf, an inhibitor of the myogenesis. Model of major postulated interactions in the patterning of the somite Sclerotome Determination of the dermatome  Specification by paracrine factor, neurotrophin-3 (NT3) and Wnt1, secreted from the notochord and the neural roof. Dermatome Determination of the primaxial myotome  Primaxial myotomes are induced by paracrine factor, Wnt1 & Wnt3a, secreted from the neural roof.  And, low level of Shh from the notochord and the neural tube floor plate.  Noggin in primaxial myotomes prevents BMP signals from lateral plate mesoderm.Determination of the abaxial myotome  Abaxial myotomes are induced by paracrine factor, Wnt1 & BMP4, secreted from the epidermis and lateral plate mesoderm, respectively.Determination of the primaxial & abaxial myotome Primaxial Abaxial • Specification and differentiation by myogenic bHLH proteins • Muscle fusion Myogenesis: the generation of Muscle Specification and differentiation by myogenic bHLH proteins  Muscle cells come from the primaxial and the abaxial somites.  Induced synthesis of MyoD protein by Pax3 transcription factor in abaxial somite.  MyoD protein also induced by Myf5 transcription factor in primaxial somite.  MyoD & Myf5 belong to the myogenic bHLH protein. Two way to activate gene expression by MyoD  First gene activations (direct)  MyoD directly activates the muscle specific creatine phosphokinase gene.  Chicken muscle acetylcholine receptor gene has two sites for MyoD-binding.  MyoD activates itself.  Second gene activations (indirect)  MyoD activates the cofactor genes for MyoD.Muscle Cell Fusion Characteristic of muscle tissue: Myoblasts align and fuse to form the multinucleated myotube cells. 1. Myoblasts stop dividing, secrete fibronectin onto their extracellular matrix and bind to it through α5β1 integrin. 2. The alignment of the myoblasts into chains. 3. The cell fusion event itself using the calcium ions and meltrins, metalloproteinase. 4. The myotube secretes interleukin-4 (IL4) and recruits other myoblasts to fuse with the tube.Conversion of myoblasts into muscles in culture Specification Conversion of myoblasts into muscles in culture Differentiation Satellite cell, putative stem cell with Pax7 for quiescent. Conversion of myoblasts into muscles in culture Aligned cell Aligned cell • Endochondral ossification • Tendon formation: The syndetome Osteogenesis: The Development of Bones Osteogenesis, bone formation Osteogenesis: The transformation of preexisting mesenchymal tissue into bone tissue.  Intramembranous ossification: The direct conversion of mesenchymal tissue.  Endochondral ossification: The mesenchymal cells differentiate into cartilage, which is later replaced by bone. Intramembranous